Urea (also termed carbamide) is the chief nitrogen containing end product of the animal protein metabolism in addition to being synthesised on a large scale for use as a fertiliser and as a raw material in the manufacture of urea based plastics. Further applications of urea include use in the manufacture of drugs and importantly here for melting ice on runways, driveways, paths and the like.
When used within fertilisers and de-icing compositions it is know to store and dispense urea in the form of granules, including specifically prills, such granules or prills being dispensed onto an agricultural field or iced road via a mechanical spreading device such a device commonly being towed by a vehicle.
A first problem associated with the use of urea based granules is their tendency to cake (the sticking together of neighbouring granules) resulting in a large mass of undispensable urea. Caking of urea granules is particularly acute when stored in a humid environment, water being the contributor to the observed caking phenomenon. Within the art there are various attempts to solve the problem of urea granules caking, for example U.S. Pat. No. 3,299,132 discloses a process of tumbling a bed of urea within a pre-set temperature range for a pre-set time. Effectively, the urea granules are baked so as to provide a hardened outer surface, the resulting granules exhibiting reduced caking tendency. WO 95/21689 discloses a method for producing a free-flowing or non-caking granule urea in the presence of a conditioning agent. The conditioning agent being a diralent metal oxide such as calcium oxide, magnesium oxide or zinc oxide. Further additions include the use of a granulating aid being a trivalent metal salt such as aluminum or ferric sulphate. Further, U.S. Pat. No. 3,544,297 discloses an anti-caking treatment for urea using polymerised resins. The method disclosed uses a finely divided plastic material of a thermal-setting or thermal-plastic type added to the urea granules. The plastic material provides abrasive entities inter-dispersed within the urea granules so as to break down and/or prevent caking. CA 1,146,973 identifies various attempts to reduce caking including the addition of aqueous formaldehyde solution or of aqueous formaldehyde-urea condensates such as di-and trimethylol urea as disclosed in DE-OS 2,139,278 and DE-OS 2,825,039. However, use of formaldehyde solutions is in itself unsatisfactory as the production of urea granules involves the removal of water, water being present within the formaldehyde solutions. CA 1,146,973 according to its primary teaching discloses a method of treatment of urea granules by various additives including dicyandiamide so as to prevent granule caking.
A second problem associated with the treatment of urea granules so as to reduce granule caking when in storage is the reduction of the granules ice-melting property when in use. For example, of the prior art identified above U.S. Pat. No. 3,544,297, WO 95/21689 and CA 1,146,973 provide additives to or methods of treatment of granule urea for use in fertilisers to prevent granule caking in storage. The inventors have found that such prior art additives to or treatments of urea granules in order to reduce caking, being specific to the use of granule urea for fertilisers, reduces the ice-melting property of the urea granules below an exceptable effective level in order to de-ice a road, driveway, path or the like. Conversely, prior art associated with the ice-melting application of urea are directed to improving the ice-melting property whilst not addressing the problem of granule caking during storage, U.S. Pat. No. 5,482,639 being such an example.
A third problem associated with prior fertiliser specific anti-caking disclosures is the corrosive nature of the treated urea or the depositing of unwanted residues following the de-icing process. Moreover, the deposition of corrosive compounds following the ice-melting process is also common to prior art methods of urea based ice-melting formulations as disclose in U.S. Pat. No. 5,482,639.
What is required therefore is a urea based granule blend configured with enhanced ice-melting property when in use whilst retaining reducing granule caking properties when in storage. Further, there is a need for such a granule blend having said properties which is non-corrosive and does not deposit unwanted additives following ice-melting.